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In order to save molybdenum (Mo) in chromium - molybdenum steels for automobile components, medium carbon - boron steels were investigated. Boron is not a new alloying element for structural steels, however, to date boron steels have not been widely used because of their unstable hardenability and poor machinability. Therefore, in this paper, the optimum content of boron was reexamined, and also the appropriate addition of titanium as a stabilizer of boron was investigated from the view point of hardenability. Furthermore the upper limit of manganese (Mn) content was studied to keep good machinability. The new steel grades, established on the basis of the above fundamental research, have been used on vital components of passenger cars.